Internal Ballistics

Firing a Cartridge

Accuracy is the theme of this section of the Hornady Handbook; what accuracy is, where it comes from, and how the shooter can improve it. We'll demonstrate in the following pages that it's possible for the shooter to do something about accuracy problems if he understands their origins and takes the right steps to correct them.

Factory ammo is, and must be, manufactured to specifications which will insure its functioning satisfactorily despite a wide range of chamber tolerances in a variety of bolt, slide, lever, autoloading, and single shot rifles, and revolvers and pistols both domestic and foreign. Factory ammunition must necessarily forego some accuracy potential for the individual rifle, to perform satisfactorily in all the different rifles in which it may be used. [Photo: reloading_2.jpg]

Handloads, however, can be made up for the individual firearm and its chamber without compromise and so can bring out its best capabilities.

Handloading your own ammunition - ammunition of excellent quality, custom tailored to your own rifle or handgun, and your own shooting needs - could hardly be easier or safer. It requires only four different components (empty cases, primers, powder, and bullets) and this basic equipment: a powder measure, scale, a reloading press, a set of dies, some case lube, and a loading manual. Elsewhere in this book we present detailed step-by-step procedures for reloading rifle and pistol cartridges - plus most of the data required by both the beginner and the experienced reloader, regardless of the caliber of gun or type of target for which they're loading. Our purpose here is to help clarify certain aspects of shooting and reloading which are frequently unknown or misunderstood. The more secure your knowledge of these complex factors, the better will be your chances of obtaining the ultimate in accuracy and performance from your firearm and ammo.

To explain what actually happens in the chamber of a rifle or handgun when it is fired, how the case and bullet are affected, we will employ illustrations with exaggerated clearances which would otherwise be difficult to see.

The rimless cartridge shown here (above, left) is a new factory round which fits the chamber rather loosely; its bullet is not in perfect alignment with the bore; and the case doesn't contact the front of the chamber (above, second from left). As the firing pin strikes, it moves the case forward to contact the front of the chamber (above, third from left), giving a little headspace (above, right) - but not a dangerous amount.

Notice that the violence of its flash backs the primer part way out of the pocket (left) - and as the powder is ignited and pressure builds up, the brass case expands to fill the chamber completely (middle), preventing any escape of gas to the rear.

As the pressure continues to build, the case is forced so tightly against the chamber wall that it cannot move; but since we had a gap between the base of the cartridge case and the face of bolt or breech block - what we termed a little headspace - the case itself must stretch in the head region circled (above, right) to force the case head back.

As the case is moved rearward the primer is reseated in its pocket, when the bullet exits up the barrel the pressure drops, the case cools, and the brass contracts enough to permit extraction of the fired cartridge case from the chamber (left).

The concept of headspace is one every shooter should understand fully, both in the interest of more accurate shooting and personal safety. In the Illustrated Glossary at the back of the 8th Edition of the Hornady Handbook we summarize four different headspacing systems and define the term very simply as "the fit of a cartridge in a chamber measured as the distance from the breech face to that part of the chamber which stops the case's forward movement." As we have just seen with this rimless case, an imperfect fit of the cartridge in the chamber will result in case stretching in the critical head region. Fired in a chamber having considerable headspace, this 308 Winchester case (right - closeup) has had its wall thickness substantially reduced at the critical head region - and would almost certainly have separated on the next firing - as did this case (far right). Such a rupture might permit gas to escape rearward through the action, endangering the safety of the shooter.

Next we'll illustrate another type of case with a different head-spacing system; the rimmed case. When the primer in this case is struck by the firing pin (left, top), the case itself cannot move forward appreciably because it is the rim and not the shoulder which stops its forward movement and thus positions it in the chamber. As the powder is ignited and pressure builds inside the case, its thin walls readily expand to contact the chamber; but because the rimmed head is virtually in contact with the bolt face or breech block the case (left, middle) will not stretch as much in the critical head region as did the rimless case we first illustrated - unless the action used is an exceptionally weak one.

The belted case - often used in so-called "Magnum" cartridges - is a third type of design frequently encountered. The "belt" is in effect a rim moved to the front of the head rather than being at the rear. When this type of cartridge is struck by the firing pin it is moved forward only slightly, regardless of the space at the shoulder. When its has been fired (left, bottom) there is very little stretch in the critical head region; however, if headspace is not close the case will stretch just above the belt.

A cartridge case plays a demanding role in the process of firing. It must contain hot gases under immense pressures and seal the chamber to prevent their rearward escape. Despite safety features incorporated in modern rifle actions, case failure resulting from faulty headspacing poses possible dangers to the shooter. As we'll show, the careful reader who understands the concept of headspace can take steps to minimize the risks of case separation.

To review our discussion up to this point, we've established that factory ammunition, manufactured to function in a wide variety of actions and chambers of varying dimensions, will be an imperfect fit in the individual rifle and handgun. The bullet will not necessarily be in perfect alignment with the bore; and the case will expand on firing to conform to the dimensions of the chamber when the cartridge is fired. If headspace is not close there will be some stretching or thinning of the case wall in the process.

But while our case conforms perfectly to the chamber in which it has been fired, one important dimension has been changed so that it cannot be reused as is: the case neck has expanded to release the bullet and is now too large to hold a new bullet securely. This brings us face to face with a decision that plagues and confuses many reloaders: whether they should full-length resize their cases, returning them to standard dimensions - or whether they should resize the necks only.

Let's examine the process of neck sizing to see what advantages it affords. Most cases can be neck sized in a normal full length die just by unscrewing it slightly, though a proper neck sizing die is preferred. Since the body of the case will not be worked in the die, it is not necessary to lubricate the full case. It is necessary only to dip the neck lightly in a dry lubricant, such as powdered graphite, to prepare for the resizing operation. A new primer, some new powder, and a new bullet get the case ready for reuse.

Though some rifles deliver their best groups when full length resized, neck sizing alone usually promotes better accuracy, because when our reloaded cartridge is returned to the chamber it is almost a perfect fit; headspace is just right with all cases, whether rimmed, belted, or rimless; and most helpfully of all, the new bullet is almost perfectly aligned with the bore.

Observe what happens when this reloaded cartridge is fired: the striker does not drive the case forward because the shoulder is already in virtual contact with the chamber (left, top); and headspace is minimum.

The case is not subject to stretching in the critical head region (right, top) as it was when fired originally.

Some reloaders may wonder why it would not be possible to adjust a full length size die to eliminate excess headspace and obtain the advantages we've just noted for the process on neck sizing only. To get the answer to this question we'll return to our cutaway illustrations. The full length sizing die, though adjusted for minimum headspace, "works" the brass, eventually making it flow into the neck area thereby lengthening the case (right, bottom).

When the reloaded cartridge is returned to the chamber, it doesn't have excess headspace - but it's almost as loose a fit as it was when brand new (left, middle). One benefit full length resizing provides (if done with minimum headspace) is in aligning the bullet with the bore (left, bottom), though some misalignment may still be possible due to case or chamber eccentricities.

Repeated full length resizing will keep lengthening the case until it must eventually be trimmed, possibly after only three or four rounds. The reasons for this are simple. The case body expands on firing; and its diameter is reduced in full length resizing. The brass displaced must go somewhere - and it does; it is pushed upward to lengthen the case.

When an overlength case is chambered, the mouth or edge of the neck will come up against the throat (left, top) before the bolt has fully closed or the case shoulder has contacted the chamber (left, upper middle). The camming action of the bolt is so powerful that it will actually crimp the case mouth fully into the bullet (left, lower middle) and wedge the case so solidly between the bullet and the throat that the neck cannot expand to release the bullet. Chamber pressures in this situation can and most certainly will go dangerously high (left, bottom).

There will be ample evidence to confirm the existence of dangerous pressure levels directly attributable to the overlength cases. The shooter may get belted sorely in the cheek; the bolt may have to be hammered open; and case head may present clear indications of too much pressure (right, top). The primer will be excessively flattened; there will be a crater around the firing pin indentation; and brass may have been extruded into the ejector slot.

If the case head is miked (right, bottom) it may show expansion - and even half of one thousandth of an inch (.0005") increase in the diameter of the case head is an indication of high pressure.

Case lengthening produced by repeated full length resizing will shorten case life. Continuous working of the brass between chamber and die stretches it, eventually producing cracks in the case and finally complete head separation. Hot loads, even if necks are trimmed as called for, speed up this process.

If optimum accuracy and prolonged case life are important to the reloader, our advice is to neck size alone whenever practical. There are of course, situations in which it is advisable to full length resize. Shooters reloading for pumps, lever-actions, and autoloaders must do so to facilitate reliable chambering. A lever-action, for example, doesn't have the powerful camming action of a bolt action and may not easily chamber cases larger than factory standards. Shooters who reload cases from one bolt action for another will also find it necessary to full length resize. Even shooters reloading for one bolt action will occasionally need to full length resize for the sake of easy clearance; over repeated firings the case may conform more and more tightly to chamber dimensions, making chambering and extraction increasingly difficult.

While the issue of cartridge case to chamber fit is of vital importance to the subject of accuracy, bullet seating has a considerable effect on accuracy in many rifles - and on pressure and velocity as well.

The section ahead of a rifle's chamber just before the rifling starts (above) is called the "leade," throat, or "freebore" and is really the bore of the barrel with the rifling reamed away. As our bottom three illustrations show, it varies considerably in length and in the angle at which the rifling is cut, depending on the views of different factories, cartridge designers, and barrel and gun makers. Normally it is quite short and the rifling starts perhaps only one 32nd of an inch from the bullet; but in some rifles the freebore might allow over half an inch of bullet travel before it contacts the rifling firmly.

To illustrate the effects of variations in bullet travel before the bullet enters the rifling, we'll compare a standard load with adjustments made only in the bullet's seating depth.

In a "normal" load with the bullet seated to allow about one 32nd of an inch gap (A) between the bullet and the initial contact with the rifling, pressure builds very smoothly and steadily even as the bullet takes the rifling. Pressure remains safe throughout the powder burning period (B), and the velocity obtained - 3500 fps - is "normal" for this load in this rifle.

Seating the bullet deeper to allow more travel before it takes the rifling, as in these next two illustrations, permits the bullet to get a good running start (C). Powder gases quickly have more room in which to expand without resistance, and their pressure thus never reaches the "normal" level. Nor does the velocity; with the same powder charge it only comes to 3400 fps (D).

When the bullet is seated to touch the rifling, as in the accompanying illustrations, it does not move when the pressure is low (E); and not having a good run at the rifling as did the other bullets, it takes greatly increased pressure to force it into the rifling. As the rapidly expanding gases now find less room than they should have at this time in their burning, the pressure rise under these conditions is both rapid and excessive (F). Velocity is high at 3650 fps - but at the expense of rather dangerous pressure. Many rifles deliver their best groups when bullets are seated just touching the rifling. Seating bullets thus can be done quite safely if the reloader will reduce his charge by a few grains. The lighter load will still produce the "normal" velocity without excessive pressure.

This brings up another pointer on accuracy for shooters who may have a few thousand rounds through their rifle barrel and have noted a fall off in the accuracy they can obtain with their standard loads. Hot gases from the shots previously fired through the barrel erode the throat and thus increase the distance a bullet must travel before contacting the rifling. By loading longer bullets and seating them farther out so they'll touch the rifling - making powder charge adjustments as necessary, of course - accuracy can often be improved.

The number of factors affecting the performance of a cartridge is remarkable; we've looked at the cartridge to chamber fit; alignment of the bullet with the bore; the depth to which a bullet is seated and the amount of travel it undertakes before contacting the rifling. The primer, a component many reloaders take for granted, may influence performance if it's not up to the task of igniting densely packed smokeless powder. Some powders burn more uniformly than others and contribute to better accuracy.